Role of temperature in determining relative abundance in cave twilight zones by two species of lungless salamander (family Plethodontidae)

Abstract

Lungless salamanders of the family Plethodontidae have historically been considered to be passive conformers to their surrounding thermal environment because there is no evidence that they thermoregulate behaviourally in the field. In contrast, plethodontids readily choose optimal temperatures when placed on experimental thermal gradients. It has been hypothesized that restriction to moist habitats prevents these salamanders from exploiting thermally diverse microhabitats in nature. We tested this hypothesis, as well as the hypothesis that response to temperature differs among plethodontid species, by investigating the thermal ecology of two species (Cave Salamander, Eurycea lucifuga Rafinesque, 1822, and Northern Slimy Salamander, Plethodon glutinosus (Green, 1818)) occupying twilight zones of six caves in northwestern Georgia. We recorded inside and outside temperatures, as well as the number of each species, for each of three seasons (summer, fall, spring) over 13 years. We also tested for differences in thermal preference along experimental gradients in the laboratory. We further generated environmental niche models (ENMs) to investigate the potential role of abiotic variables, including environmental temperature, in determining the geographic range of each species. We found that both species responded to cave temperature in such a way as to suggest that these salamanders thermoregulate behaviourally when given a diversity of thermal options within a relatively constant moisture regime. We also determined that E. lucifuga prefers lower temperatures than P. glutinosus. ENM analysis indicated that, while abiotic variables both strongly influence the ecological niche of both species, the range of E. lucifuga is strongly predicted by them. The geographic distribution of P. glutinosus is apparently heavily influenced by the presence of closely related, contiguous neighbors with similar niche requirements.